SF bug 705836: struct.pack of floats in non-native endian order
pack_float, pack_double, save_float: All the routines for creating
IEEE-format packed representations of floats and doubles simply ignored
that rounding can (in rare cases) propagate out of a long string of
1 bits. At worst, the end-off carry can (by mistake) interfere with
the exponent value, and then unpacking yields a result wrong by a factor
of 2. In less severe cases, it can end up losing more low-order bits
than intended, or fail to catch overflow *caused* by rounding.
Bugfix candidate, but I already backported this to 2.2.
In 2.3, this code remains in severe need of refactoring.
diff --git a/Misc/NEWS b/Misc/NEWS
index baa3da3..3fb44c1 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -37,6 +37,14 @@
Extension modules
-----------------
+- The platform-independent routines for packing floats in IEEE formats
+ (struct.pack's <f, >f, <d, and >d codes; pickle and cPickle's protocol 1
+ pickling of floats) ignored that rounding can cause a carry to
+ propagate. The worst consequence was that, in rare cases, <f and >f
+ could produce strings that, when unpacked again, were a factor of 2
+ away from the original float. This has been fixed. See SF bug
+ #705836.
+
- New function time.tzset() provides access to the C library tzet()
function, if supported. (SF patch #675422.)